🐛 Unexpected jitter on MenuFlyoutSubItem appears

[TENX-S]

When the MenuFlyoutSubItem appears, it quickly moves a short distance to the lower right corner.

bug.mp4

[TENX-S]

This behavior can be confirmed in the example app too.

[bdlukaa]

This is probably because of the following lines:

fluent_ui/lib/src/controls/flyouts/content_manager.dart

Lines 63 to 72 in 9c0840d

	@override
	void initState() {
	WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
	final context = _key.currentContext;
	if (context == null) return;
	final box = context.findRenderObject() as RenderBox;
	setState(() => size = box.size);
	});
	super.initState();
	}

The sub items doesn't get to know the box size at the very first frame, causing this jitter.

[TENX-S]

Yeah. I think that is the root cause. I build and run the example without the post frame callback, and the jitter is gone. But the FlyoutListTile size doesn't get right:

[bdlukaa]

Yeah. To fix this, we'd need to make use of Flutter box system - which is kind of complex. We already do it for other widgets, like ComboBox.